Electrodes are used to connect medical electronic equipment to patients. One type of medical electronic equipment frequently attached to a patient is stimulation equipment. Stimulation equipment has several different uses, such as stimulating a patient's heart to beat (defibrillation), stabilizing the heart beat of a patient (cardiac pacing), transcutaneous nerve stimulation for pain control and other uses. The stimulation equipment delivers one or more electrical pulses via the electrodes to the skin of the patient.
Most electrodes used with medical electronic equipment incorporate an electrically conductive, impedance-decreasing gel disposed between a flexible conductive plate and the patient's skin. Typically, the conductive plate is made of a metal foil, such as a tin alloy, or a conductive plastic impregnated with carbon. The gel serves to ensure good electrical contact between the patient and the plate, and to adhere the electrode to the patient's skin. The non-skin-facing side of the plate is generally covered with an electrically insulating backing layer. Usually, a post electrically coupled to the plate, projects from the insulating backing layer. Wire is connected to the post from the medical electronic equipment for supplying electrical pulses to the electrode.
A problem with the use of such gels is that the electrical pulses transmitted through the electrode to the patient's skin causes hydrolysis of the gel. The problem is exacerbated with medical stimulation equipment used for defibrillation or cardiac pacing because these types of stimulation equipment usually deliver higher voltages and currents to the patient, which increases the rate of hydrolysis. For example, defibrillation equipment typically delivers up to 5,000 volts to the patient at a maximum current of 60 amps. Cardiac pacing equipment commonly delivers up to 300 volts to the patient at a maximum current of 0.2 amps.
The hydrolysis produces hydrogen and oxygen gas, which tends to accumulate between the gel and the flexible metal plate and creates two primary effects that are undesirable. First, the accumulation of the gases generally decreases the conductivity between the electrode and the patient.
Second, the decrease in conductivity is not uniform across the surface of the electrode. The gas commonly accumulates in pockets, such that the conductivity between the electrode and the patient substantially decreases in areas of the electrode that are separated from contact with the patient by a gas pocket. To maintain the same current flow to the patient an increased current flow occurs in other areas of the electrode where conductivity has not been decreased. The increase in current flow in other areas of the electrode increases the current density in these areas. If the density of current flow increases enough in these areas, patient discomfort results. If the current density increases even further, burning of the patient's skin may result. The problem is exacerbated with electrodes designed for pediatric use because these electrodes tend to be smaller, yet have current flows comparable to electrodes used for adults.
There is a continuing need for a solution to these problems. U.S. Pat. No. 4,300,575 discloses an electrode including air-permeable components so that the patient's skin can "breathe" through the electrode. U.S. Pat. No. 4,367,755 discloses an electrode including a multiplicity of perforations formed therethrough, such that moisture absorbed by the electrode from the patient's skin may be dissipated through the perforations.
While perhaps satisfactory for some uses, the electrodes disclosed in the above two patents are generally unsatisfactory to solve the problem caused by hydrolysis of the gel as described above. Specifically, in the electrodes disclosed in U.S. Pat. Nos. 4,300,575 and 4,367,755, electrically conductive gel can pass through the perforations, or through the gas-permeable layers to the upper surface of the insulating backing, which results in an electrical hazard to medical personnel.
More particularly, medical personnel treating the patient could inadvertently come into contact with gel on the upper side of the insulating layer and be electrically shocked. This problem is of special importance with electrodes used for cardiac pacing or defibrillation because of the higher electrical voltages and currents used with these types of electrodes. In general, it is inadvisable for medical personnel to physically contact a patient who is undergoing cardiac pacing or defibrillation with externally applied electrodes. Nonetheless, gel present on the upper side of the insulating layer of the electrodes creates an even greater hazard because the gel tends to make a good electrical connection between the electrode and anything that contacts the gel.
Accordingly, the present invention provides a solution to the above described problems.